IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009 4033

Fabrication and Magnetization Reversal Processesfor Co/Cu Multilayer Nanowires

R. Sharif���, X. Q. Zhang�, M. K. Rahman�, S. Shamaila���, J. Y. Chen�, X. F. Han�, and Y. K. Kim�

Institute of Physics, Chinese Academy of Sciences, Beijing, ChinaPhysics Department, University of Engineering and Technology, Lahore, Punjab, Pakistan

Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea

Co/Cu multilayer nanowires fabricated in an array using anodized aluminium oxide (AAO) template has been investigated. Experi-mental conditions are optimized to fabricate Co/Cu multilayer systems with fixed Cu and variable Co layer thicknesses. Magnetizationreversal mode is found to depend on the Co layer thickness. A transition occurs from coherent rotation to a combination of coherentand curling rotation at around ����� � �� nm with increasing t(Co). The reversal modes have been investigated using the magnetichysteresis loops measured at room temperature for Co/Cu nanowires placed at various angles between the directions of the nanowireaxis and external fields using a vibrating sample magnetometer. The magnetic easy axis changes from the direction perpendicular tonanowires to that parallel to the nanowires at around ����� � �� nm, indicating a change in the magnetization reversal mode. Thereversal mode for the nanowires with thin disk-shaped Co layers is of a coherent rotation type, while that for long rod-shaped Co layerscan be explained by a combination of coherent rotation and a curling mode.

Index Terms—Coercivity, magnetization reversal.

I. INTRODUCTION

D URING the last decades, remarkable progress has beenmade on fabrication of nanostructured magnetic materials

due to their technological applications, particularly for mag-netic and spintronic devices such as high-density data storageand magnetic field sensors. Among various nanoscale magneticmaterials, magnetic/nonmagnetic multilayer nanowires are ofparticular interest because of their unique magneto-transportproperties in the current perpendicular-to-plane (CPP) ge-ometry. Ferromagnetic nanowires exhibit unique and tunablemagnetic properties due to the inherent shape anisotropy andthe small nanowire dimensions. The magnetic properties ofnanowires can further be tuned by tailoring the shape of theferromagnetic component in the multilayer nanowires. Thegeometry of the nanowires with layer interfaces perpendicularto their long axis, make them suitable for the CPP magne-totransport measurements [1]. Although the phenomenon ofgiant magnetoresistance (GMR) effect in the CPP geometryhas been well studied [2]–[6], little has been investigated in themagnetization reversal mechanism in the multilayer nanowiresystems. As the experimental determination of the magnetiza-tion reversal mode for nanoscale magnetic objects remains achallenge for researchers, most of the research works have beenfocused on the computational modeling [7]–[14]. For the mag-netization reversal of nanoscale magnetic objects few modelshave been proposed, e.g., coherent rotation, curling, buckling,fanning, and nucleation of reversed domains. For presentwork, the coherent rotation and curling models are consideredfor the magnetization reversal of the Co/Cu nanowires sincethese models are among the most suitable for nanostructuredmagnetic objects, particularly nanowire systems [15], [16]. For

Manuscript received March 06, 2009; revised May 28, 2009. Current ver-sion published September 18, 2009. Corresponding author: R. Sharif (e-mail:rsharif2002@yahoo.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2009.2026286

TABLE IDECREASE IN Co LAYER THICKNESS WITH INCREASING Cu DEPOSITION TIME

present report, Co/Cu multilayer nanowires are grown by elec-trodeposition in anodized aluminium oxide (AAO) templates.The objective of present research is to systematically studythe change in magnetic anisotropy and magnetization reversalmechanism in Co/Cu multilayer nanowires as a function of theCo layer thickness t(Co).

II. EXPERIMENTAL METHODS

Co/Cu multilayer nanowires are grown by electrodepositionusing a commercial alumina template (Whatman) with an arrayof holes nm in diameter and thickness of m inlength. A conducting layer of copper was coated on one sideof each template that served as a working electrode for elec-trodeposition. An electrical contact is made to the conductinglayer. The electrolytic solution contains 1.5 M CoSO , 0.025 MCuSO , and 0.5 M H BO with the pH value of 3.5. The mul-tilayer nanowires are electrodeposited using a pulsed potentialtechnique by periodically switching the deposition potential be-tween and V for deposition of the Cu and Co layers,respectively. When the low current was applied only Cu wasdeposited, while in high current case Co with some Cu inclu-sion was inevitable, as manifested by the inductively coupledplasma atomic emission spectroscopy (ICP-AES) measurement,with the concentration of Co nanowires more than 95 wt % Co.

Table I shows the decrease in Co layer thickness with in-creasing Cu deposition time. It gives us an idea to design ourexperiment of multilayer nanowires with various thicknesses

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4034 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009

Fig. 1. Variation in Co layer thickness due to variation in Cu deposition time.

of t(Cu) and t(Co). The values given in the Table I are plottedand shown in Fig. 1. A longer Cu pulse means more Co wouldbe sacrificed by the metal exchange reaction because Cu is amore noble metal than Co [17]. It means that longer Cu depo-sition causes higher Co dissolution into an electrolyte duringthe Cu deposition time. Note that the Co and Cu layer thick-nesses did not linearly change with the deposition current. Firstof all, the conditions of Co and Cu reduction were verified, andconfirmed the corresponding charge consumption and thicknessof reduced metals. The thickness of each layer is controlled byoptimizing the pulse duration and the cumulative charge trans-ferred during each pulse. The actual thickness of each layer isdetermined by scanning electron microscopy (SEM). In orderto clearly identify the individual layers in the SEM images andto estimate their growth rate, multilayer nanowire samples withthe thickness of each layer greater than that of the samples usedfor magnetic characterization were also prepared. For example,

nm nm were prepared based upon ourexperiments of the cumulative charge transferred during eachpulse as it is hard to observe these small thicknesses using SEM.Therefore, the tailoring of Co/Cu nanowires was achieved withdesired thicknesses in a reproducible manner by controlling de-position time and current density. Magnetic characterization ofthe nanowire samples was carried out without removing the alu-mina templates. The thickness of the Co layers t(Co) in the mul-tilayer nanowires was varied in a range 30 nm–10 m with theCu layer thickness t(Cu) keeping constant at 15 nm. To studythe magnetization reversal mechanism of the Co/Cu nanowires,the magnetic hysteresis loops were measured at room tempera-ture in an applied field of up to 15 kOe using a vibrating samplemagnetometer (VSM) at various angles between the directionof the nanowire axis and the field ranging from 0 to 180 insteps of 10 . For TEM analysis, the templates were completelyremoved using 1 M NaOH solution for 12 h and then the sampleswere thoroughly washed with water. After washing, the sampleswere dissolved in ethanol so that the nanowires become wellseparated from each other. A drop of that solution was placedon a copper grid to study using TEM. For SEM analysis, thesamples were fixed on copper tape and then partially removedtheir templates using 1 M NaOH solution for 4 h.

Fig. 2. Schematic of rod-shaped and disk-shaped multilayer nanowire arrays.

Fig. 3. (a) SEM and (b) TEM images of the rod shaped multilayer nanowires.

III. RESULTS AND DISCUSSION

Fig. 2 shows the schematic picture of the Co/Cu multilayernanowire system with variation in the thickness of Co layerfrom solid-rod shape to thin disk shape. Scanning electron mi-croscopy (SEM) and transmission electron microscopy (TEM)images are shown in Fig. 3(a) and (b), respectively. Fig. 4 showsthe magnetic hysteresis loops measured at room temperature forCo/Cu nanowires with nm (a) mand (b) nm in magnetic fields applied perpendic-ular and parallel to the nanowire axis. As t(Co) decreases, theeasy axis of the nanowires changes from the direction parallel tonanowires to that perpendicular to the nanowires. The Co layersin the multilayer nanowires with m and 30 nmare formed in a rod-like shape and disk shaped respectively. Infirst case, the saturation field measured along the nanowire axis

is smaller than that for the directions perpendicular tothe axis , indicating that the magnetic easy axis liesin the direction parallel to the nanowires, as shown in Fig. 4(a).In the second case, the magnetic easy axis lies in the directionperpendicular to the nanowires, as shown in Fig. 4(b). A transi-tion for the direction of the magnetic easy axis occurs at about

SHARIF et al.: FABRICATION AND MAGNETIZATION REVERSAL PROCESSES FOR Co/Cu MULTILAYER NANOWIRES 4035

Fig. 4. Magnetic hysteresis loops (a) for rod shape and (b) for disk shapenanowires.

nm, which corresponds to a ratio of ,where t and d, respectively, represent the thickness of the Colayers and the diameter of the nanowires.

Since the value for the transition is much smallerthan , the shape anisotropy is not negligible so that bothmagnetocrystalline anisotropy and shape anisotropy should beconsidered in evaluating the direction of the magnetic easy axis.The XRD analysis (not shown here) of the multilayered Co/Cuarray revealed that this material possessed fcc polycrystallinestructure with ( ) orientation.

As Co layers have a ( ) texture along the nanowires andpure fcc-Co is known to have an easy axis along thedirection, it is reasonable to assume that the easy axis of fcc-Coalso lies in the direction. It is therefore suggested that themagnetocrystalline anisotropy of the ( )-textured Co layerstends to have the easy axis along the nanowire axis. When

, the easy axis lies along the nanowire axis since boththe shape anisotropy and magnetocrystalline anisotropy tend toalign the easy axis along the nanowires. When , theeasy axis lies in the direction perpendicular to the nanowiresbecause there is competition between shape anisotropy andmagnetocrystalline anisotropy but shape anisotropy dominatesover the magnetocrystalline anisotropy in this range of theaspect ratio and the c-axis lies perpendicular to the nanowiresaxis.

Coercivity is an important ferromagnetic material propertyand understanding of it can provide us insight into magnetiza-tion reversal processes [15]. In the case of magnetic nanowires,the magnetization reversal process and coercivity sensitivelychange with the angle between the field and the nanowire axes.Therefore, the angular dependence of coercivity is correlated tothe magnetization reversal as different reversal modes are ex-pected to show different trends in the angular dependence ofcoercivity. To gain insights into the reversal in Co/Cu nanowires

Fig. 5. The values for � have been extracted from hysteresis loops measuredfor the nanowires are plotted as a function of � for Co/Cu nanowire sampleswith (a) ����� � � �m and ���� � � nm and (b) ����� � � nm and���� � � nm.

with rod-shaped or disk-shaped Co layers, the variation of co-ercivity where the values for have been extracted fromhysteresis loops measured for the nanowires and plotted as afunction of for Co/Cu nanowire samples with mand nm [Fig. 5(a)] and nm and

nm [Fig. 5(b)] respectively.Coercivity Hc for coherent rotation decreases rapidly with in-

creasing angle whereas for curling the value of Hc increaseswith increase of angle between the field and the easy axis ofthe magnetization. The easy axis of magnetization for

m nanowires lies in the direction parallel to the nanowireaxis. Hc has been observed to increase from 0 to 40 [seeFig. 5(a)], indicating that magnetization reversal in Co layersoccurs in a curling mode. When is further increased from 45to 90 , Hc decreases swiftly and reaches a minimum at around

, due to a change in the magnetization reversal froma curling to coherent rotation mode at around repre-senting an M-type curve. The results obtained for the Co/Cumultilayer nanowire system suggest that magnetization reversaloccurs by curling for and by coherent ro-tation for . Han et al. [15] and Goolaup etal. [16] have also reported similar M-type results on Ni mag-netic nanowires, showing that curling reversal is present forsmaller and coherent rotation at larger . Therefore, it can beconcluded that multilayer nanowires with rod-shaped magneticlayers show a reversal behavior similar to that of single elementnanowires. The dipole-dipole interactions tend to align the mag-netization of the rod-shaped layer along the nanowire axis with

4036 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009

different poles facing each other so that the magnetic behaviorof the multilayer nanowires is similar to that of single-elementnanowires with large values for t/d. Since the coercivity Hc in-creases for curling with increasing angle between the direc-tions of the applied field and the easy axis, while Hc decreaseswith for coherent rotation [15]. The easy axis for nanowireswith disk-shaped Co layers lies in a direction perpendicular tothe nanowire axis as shown in Fig. 4(b) for nm.Magnetostatic interactions between adjacent magnetic layers ineach nanowire appear to modulate the magnetic behavior. Whenthe magnetic layers in nanowires are in a disk shape, the interac-tions between the layers tend to align their magnetization alongthe layer direction antiparallel to each other to give a higher sat-uration field than that of isolated disk-shaped magnetic objects.When the magnetic layers are in a rod shape, the interactionsbetween the layers tend to align their magnetization along thenanowire axis with different poles facing each other, thus themagnetic layers have a magnetization reversal behavior similarto that of single-element nanowires.

IV. CONCLUSION

Fabrication of multilayer Co/Cu nanowires with well con-trolled thickness of each layer was achieved by optimizing theexperimental conditions. It has been shown that the magnetiza-tion reversal mode for Co/Cu multilayer nanowires depends onthe magnetic layer thickness. Magnetic easy axis changes fromparallel to perpendicular of the Co/Cu nanowires as Co layerthickness changed from rod-shape to disk shape. Magnetizationreversal behavior was explained qualitatively by coherent-rota-tion and curling mode.

ACKNOWLEDGMENT

This work was supported by the State Key Project of Fun-damental Research of Ministry of Science and Technology

(MOST, No. 2006CB932200 and 2009CB929203) and Na-tional Natural Science Foundation (NSFC, Grant 10874225,50721001, and 60871048). X. F. Han thanks the partial supportof the international joint projects of NSFC—The Royal Society(UK) and NSFC—Australia DEST and the partial support ofK. C. Wong Education Foundation, Hong Kong. The authorgratefully acknowledges the support of K. C. Wong EducationFoundation, Hong Kong.

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